Identifier

Author

Degree

Master of Science in Mechanical Engineering (MSME)

Department

Mechanical Engineering

Document Type

Thesis

Abstract

A bimetallic recurve device was designed, fabricated and tested as a temperature sensor. The device is to be used for sensing temperatures up to 300 C inside oil wells for downhole condition monitoring. Continuous downhole measurements at high temperatures and pressures are required to monitor conditions downhole instruments are exposed to during use. Currently mercury thermometers and resistive temperature detectors (RTD) are used for downhole temperature measurements. Microsensors have potential application downhole, due to their small size and inherent robustness. The principle of a bimetallic beam was used to measure temperature. A bimetallic beam deflects with changes in temperature due to differential thermal expansion of the two materials in the beam. Analytical and numerical models were developed for parametric analysis of recurve thermomechanical elements. Invar was electrodeposited with the desired ratio through the depth of the structures of Ni to Fe as 36%:64%. Prototype recurve structures were fabricated using the LIGA microfabrication process. X-ray masks were designed and fabricated to improve the alignment process. A substrate containing twelve different devices was fabricated using a two mask process, with eleven of the structures successfully released. Recurve structures 8 mm long with a 500 µm X 100 µm cross-sections were tested using a dynamic mechanical analyzer. The prototypes were loaded at room temperature and a heating rate of 10 C/min was applied in 20 C steps, with a one minute hold at each step for deflection measurement, until the temperature reached 300 C. The deflection was measured using a probe tip resting against the sample with a force of 0.0001N. Deflection increased linearly to a maximum deflection of 65 µm at a temperature of 200 C. With further increase in temperature, the deflection was found to decrease linearly. The Curie effect was hypothesized to influence this behavior of the Invar alloy. The analytical model of the bimetallic recurve beam with the given experimental conditions and nominal dimensions predicted a maximum free deflection of 54.17 µm at 200 C. Differences were attributed to differences between the experimental conditions and model assumptions.